Without knowing all the details, my impression is that this is more likely to be some malfunction in the process rather than simply insufficient fuel being available. You make no mention of fat. Have you been staying high on the sweet potatoes and fruit all along? It's certainly not unusual for people to feel as you do, but it shouldn't be a struggle to eat enough, which suggests there's something else wrong. And from the endurance perspective it's your fat metabolism that you want to be strong, and if you eat constantly then you give it very little opportunity. Could you perhaps give a little mor information on what you're eating, and in particular just how it is that you're 'failing'?

Would you mind editing your post with what a day of meals is for you? Also how long you've been Paleo? Also are you supp'ing at all? Whey, etc.?

It took me months to find nutritionally what exactly I needed to stay optimum at my workouts and went from 6 meals a day down to two. But I definitely let my body decide what it wants so some days its 2+. For reals, N=1 it seemed forever until I got it right. I never used to be able to go in fasted in the beginning of my Paleo/Primal transition and had banana/dried coconut/almond butter/sea salt before going in so I wouldn't flag. Now: a typical day for me is workout in a fasted state then food within 30 minutes: protein, veg, sweet potato, fruit, coffee. Dinner will be: protein, veg, tuber. Before bed: sheep milk yogurt. If there are snacks then maybe bites of dark chocolate, avocado, jerky, chopped veg, maybe a piece or two of raw cheese, etc. I will tell you now that I was literally eating 7lbs of tuber/sweet potato a week until the balance was achieved, I'm down to about 3-4 now, protein is around 3lbs a week, leafy veg 1.5lbs a week + fruit, squash, other veg.

Every body is different so what works for me may not for you but I thought it would be helpful for you to see a typical day.

Note: I workout heavy 4 days a week, lifting + CrossFit and am 5'7 123-125lbs, muscles are still growing. I've been athlete my whole life, swimming to CycloCross.

Liquid calories are a lot easier to 'overfeed' on than solids. Add in a daily smoothie, and perhaps a cream-of-vegetable homemade soup. Dress your salads heavily, and add high-calorie toppings.

If it's a lack of oomph, you may just need more carbs...don't be afraid to play around with 100-200-300 g carbs with a heavy, explosive workout schedule, as long as they're not gluten-y, primarily glucose carbs. Downing a bunch of carbs is often hard the first day, but carbs beget more carb cravings/appetite, and that doesn't lead to binge feelings for me unless there's sugar or heavy fructose in the mix, at least for me.

My friend is a triathlete. He is a vacuum cleaner on the food when he is on a long bike ride. I would say plan three of four meals a day and know ahead of time what you are going to eat. Also while bike riding you may need to suppliment with food if the ride is long. +1 for liquid foods.

I add tallow, ghee, butter, and coconut oil to as many meals as possible.

Freezing tbsp chunks (or freezer trays) worth of coconut oil makes for an easy snack.

I also cook my sweet potatoes with a TON of fat, which just soaks it up.

If you train in cold environment you can four times your normal weight without gaining weight. This is precisely how M Phelps and L Armstrong trained for their sports. In China Phelps found it difficult to find foods to meet his 18,000 calorie a day diet. He worked out 12-16 hours a day in 50 degree water to maintain it. Armstrong had an industrial freezer added to his training site and his home during his run to maintain his 12-15K calories a day.